The prevent invention relates to a hardening method and an apparatus for the visible light-curing resins, and in particular to a continuously hardening method and apparatus for the visible light-curing resins by continuously hardening the objects to be irradiated consisting of visible light-curing resins while they are moved.
Conventionally, the hardening of visible light-curing resins has been performed by a hardening apparatus, comprising an exposure stage to place an object of visible light-curing resins in an enclosure and a visible light irradiation equipment to generate the visible light. It is mostly applied for the hardening of the dental materials using composite resins in the field of dentistry.
For example, the Japanese Provisional Patent Publication Sho 62-38149 describes a hardening apparatus comprising fluorescent lamps emitting the visible light with wavelength of 400-500 nm and a stand, on which the object to be irradiated is rotatably placed, and the Japanese Provisional Patent Publication Sho 62-47354 discloses a hardening apparatus having a halogen lamp as the light source of the irradiation light and a turntable to place the object to be irradiated.
These conventional hardening methods and apparatuses for the visible light-curing resins have the following advantageous features:
1) Because the irradiation light is visible light, there is no possibility to adversely affect the clothings or the skin of the users as often seen in case of ultraviolet ray or infrared ray; 2) Because the irradiation light is visible light, there is few loss of light due to scattering in the object to be irradiated as in the case of ultraviolet ray, and the irradiated object can be hardened deep into the core. Namely, the object with larger thickness can be easily hardened; 3) Because the stand to place the irradiated object is rotatable, the illuminance on the object is distributed evenly, and the entire object can be evenly hardened.
In order to further develop the above advantageous features of the hardening method and apparatus of the visible light-curing resins, the Japanese Provisional Patent Publication Hei 1-130920 discloses a method, by which a light-curable monomer and/or prepolymer is placed in a molded frame consisting partly of the object with light transmission property to produce the molded objects for optical application such as contact lenses or lenses for eyeglasses or the molded objects for electrical and mechanical applications such as compact disk, display plate, micro-screws, gears, etc.
However, when a number of objects of visible light-curing resins are to be produced by the conventional technique as described above, the following problems arise:
1) The conventional hardening apparatus for visible light-curing resins is of batch type to harden one or more objects at one time, and it is necessary to repeatedly carry and take the objects into and from the hardening apparatus and to irradiate the objects by many times. Thus, complicated procedure and much time are required for hardening. PA0 2) Halogen lamp, xenon lamp or fluorescent lamp used as visible light source in the conventional technique have wide variations in product quality (particularly, in luminous intensity), and the variations usually range between 10-50%. PA0 3) Even when a large quantity of the hardened products are to be produced by repeatedly using the batch type hardening apparatus for visible light-curing resins based on the conventional technique, it is impossible to obtain the hardened products from the visible light-curing resins having a constant property because the thermal history differs according to each batch.
Therefore, when the halogen lamps of the same standards are used, for instance, there are the variation of 10-50% in the illuminance of the irradiation light on the object, and this leads to the wide variations in the quality of the hardened products.
Also, there is the problem of deviation in the positions of the light source in the hardening apparatus between the product batches, and the illuminance of the irradiation light on a number of objects produced in two or more batches widely differs from each other even when the batch type hardening apparatuses of the same standards are used.
Specifically, in the batch type hardening apparatus, the atmosphere temperature in the hardening apparatus is increased from normal temperature to high temperature due to the heat generated from the light source of the irradiation light or to the heat generated by polymerization reaction of visible light-curing resins. Also, when the hardened objects are taken out, the temperature is decreased to normal temperature, and the atmosphere temperature during the irradiation of visible light changes from normal temperature to high temperature and then to normal temperature. Thus, it is difficult to have the same thermal history for each batch, and this means that the hardened products produced from the batches widely differ in their physical property (such as surface hardness, mechanical strength, etc.).
When a number of the objects are distributed to two or more batch type hardening apparatuses of the same standards, the physical property of the hardened products thus produced widely differs, and the complicated hardening procedure and much labor and time are required.
In the method and apparatus to perform the hardening of light-curing resins by irradiation light, the method and the apparatus using ultraviolet light as the irradiation light has been widely known. Also widely known are the method and the apparatus to irradiate the ultraviolet light using high pressure mercury vapor lamp or arc discharge lamp as light source and to irradiate the light by continuously moving the objects made of ultraviolet light-curing resins.
However, because ultraviolet light is used as the irradiation light in this case, it is disadvantageous because of the loss of light due to the scattering of light within the objects and to the difficulty to perform polymerization hardening to the core of the thick objects because of the ultraviolet absorption of the ultraviolet light-curing resins itself. Accordingly, when ultraviolet light is used, the thickness of light-curable monomer and/or prepolymer is limited to the ink, film, coating film, etc. having thickness of several .mu.m to several hundreds of .mu.m. Namely, when ultraviolet light is used as the irradiation light, the interior of the hardened objects remains unpolymerized and satisfactory bracket cannot be obtained even if the composite resin containing inorganic filler is filled in the mold with light transmission property and the orthodontic bracket is to be molded and hardened. Even when ultraviolet light-curing resins containing coloring agents are used, hardening is performed only insufficiently, and even a color plate with thickness of about 2 mm cannot be molded.
To avoid the above problems when ultraviolet light is used as the irradiation light, the illuminance may be increased to such degree that the hardening to the deep core can be accomplished, whereas the surface of the irradiated object is exposed to extremely strong ultraviolet light in such case.
As the result, the hardened products may be thermally deformed due to extreme temperature rise, or coloring or discoloring or even ignition may occur in some cases.